A piston assembly is a key component of a hydraulic damper, and its construction and configuration has a major influence on a damper force vs. piston velocity characteristic during compression and rebound strokes of the damper. It is therefore desirable to enable for shaping and tuning of this force-velocity relation for each piston velocity range (low speed, medium speed, high speed) independently for each range and independently for the compression stroke and for the rebound stroke, in order to improve safety and vehicle handling properties, reduce unwanted vibrations, improve passengers comfort, etc.
U.S. patent application publication no. US 2010/163355, Japanese patent application publication no. JP 2000055103 and international patent application publication no. WO 2014/156445 all disclose piston assemblies provided with various kinds of X-flow (also called cross-flow) arrangements through a piston body, where the piston body is provided with a number of first channels sloped with respect to the damper axis and a number of second channels sloped with respect to the damper axis, so that a compression side of the first channels is radially outward with respect a rebound side thereof and a rebound side of the second channels is radially outward with respect to a compression side thereof. At least one disc covers the rebound side of the first channels and at least one disc covers the compression side of the second channels.
Furthermore, U.S. Pat. No. 5,148,897 discloses a pressure-operated valving arrangement for a shock absorber piston assembly provided with bi-directional primary and secondary flow paths for regulating the damping forces generated during both rebound and compression strokes. During a rebound stroke, a pressure differential is generated across a moveable valve disc which operates to regulate fluid flow from an upper portion to a lower portion of the shock absorber's working chamber. A pilot orifice in the valve disc and a bleed slot associated with a rebound blow-off assembly are sized to generate the desired pressure differential across the valve disc. Manipulation of the size ratio permits universality of design, providing economic manufacturing of the shock absorber.
U.S. patent application publication no. US 2010/294604 discloses a damping mechanism having a piston body provided with channels allowing for an “N-shaped” flow of working liquid through the piston and check valves that may open after the piston rod reaches a predetermined velocity threshold, thus enabling a flow of working liquid directly through the piston.